At various stages of the operation of a typical paper making machine a rapidly moving, continuous sheet of paper must be threaded through different parts of the machine as the paper is made. For example, one such stage occurs when the moving, continuous sheet of paper passes at high speed over a plurality of dryer rolls and then through a stack of calender rolls which calender the paper by imparting a finish of desired smoothness thereto. Since it is difficult to guide the full width of the moving paper sheet off the last dryer roll and then feed it through the calender stack, a narrow "tail" is cut to one side of the moving sheet as it comes off the last dryer roll, the balance of the moving sheet being allowed to fall into a broke pit for reprocessing. The tail (which moves at the same speed as the paper sheet from which it is cut) is more easily fed through the calender stack and, once threaded, can be used to draw the full width of the paper sheet through the calender stack.
Those skilled in the art will readily appreciate that similar threading operations are encountered at other stages in the "dry end" operation of a typical paper making machine. These may for example involve passage of the moving paper sheet onto reel systems, slitters, winders, rewinders, or other parts of the paper making machine, as will be familiar to those skilled in the art. The present application focuses upon the calender stack threading operation by way of example only. It is to be understood that the invention hereinafter disclosed may be applied to any tail threading operation encountered at the wet or dry end of a paper making machine.
Once the tail has been cut, it must be transferred to a selected part of the paper making machine. This requires that the tail be ripped or severed transversely to the direction in which it is moving, to produce an end which can be threaded through a calender stack, etc.
Various prior art techniques have been used to sever moving paper tails. U.S. Pat. No. 4,671,151 issued 9 Jun., 1987 for an invention of Sidney C. Rooney entitled "Paper Tail Cutter" discloses a cutter body member which is pivotally mounted to swing a V-shaped knife through a moving paper tail as the tail falls into a broke pit. The tail is severed in a scissor-like cutting action as the swinging knife passes a mating stationary knife. The pivoting body member supports the severed end of the tail and deflects it to a calender threader, etc. A disadvantage of such apparatus is that the body member and its associated drive mechanism are comparatively bulky, which may frustrate installation of the apparatus in cramped quarters.
U.S. Pat. No. 4,818,343 issued 4 Apr., 1989 for an invention of Paavo Laine entitled "Device For Cutting Off An Edge Strip And For Guiding The Edge Strip In Connection With The Threading Of a Paper or Board Web" discloses a pivotally mounted body member. A pair of knives are pivotally mounted on opposite sides of the body member. The knife blades normally extend parallel to and face one another. When the moving paper tail is to be severed, the body member is pivoted to position the knives on either side of the moving tail. The knives are then pivoted inwardly, such that their blades pass through the tail, severing it. The moving knives (possibly assisted by air jets) carry the severed tail end onto the body member, which directs it to a calender threader, etc. This apparatus and its associated drive mechanism is also comparatively bulky, which presents the disadvantage aforesaid. Moreover, problems may arise if the knives, fail to completely sever the tail during their single pivotal pass through the path of the moving tail.
U.S. Pat. No. 4,904,344 issued 27 Feb., 1990 for an invention of Robert E. Peiffer entitled "Automatic Web Threading Apparatus and Method" discloses a stationary serrated knife with associated means for drawing the moving paper tail across the knife to sever the tail, and for guiding the severed tail end to a calender threader, etc. Mechanical knives of this sort are quickly dulled and thus lose their ability to reliably cut the paper tail. This is particularly so if the knife is used to cut tough, hard stock such as a moving pulp sheet, which can be about 1/8" thick.
The present invention provides a compact device having a pair of counter-rotating knives which reliably sever the tail as it is drawn across the rotating knives.